A wererat is "a therianthropic creature in the shape of a rat," according to its Wikipedia page, and since it also lists Pettigrew as an example, he has to be one, right?
closed as unclear what you're asking by Jenayah, Rebel-Scum, Mithrandir, TheLethalCarrot, Edlothiad Jun 10 at 9:42
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No, he is an animagus with the power to voluntarily transform between rat and human shapes. A wererat would have similar transformation powers, but they would be a result of a curse or disease and would be governed, at least in part, by the phase of the moon. The world of Harry Potter certainly has werewolves, such as Remus Lupin, so we know that the transformation dynamics of animagi and werecreatures are entirely different.